The present invention relates generally to exposure, and more particularly to exposure apparatuses and methods, device fabricating methods, and devices fabricated from an object to be exposed or a target object. The exposure apparatus and method are used to fabricate various devices including semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, sensing devices such as magnetic heads, and image pick-up devices such as CCDs, as well as fine contact hole patterns used for micromechanics. Here, the micromechanics is technology for applying the semiconductor IC fabricating technique for fabrications of a fine structure, thereby creating an enhanced mechanical system that may operate at a level of micron.
A photolithography process uses an exposure apparatus to transfer a mask pattern onto a photosensitive material (resist) which is applied to a silicon wafer, glass plate, etc. (simply called “wafer” hereinafter), and includes steps of an application of resist, exposure, development, etching and a removal of the resist. For the exposure in this series of steps, resolution, overlay accuracy and throughput are three important factors. The resolution is the minimum size for a precise transfer. The overlay accuracy is precision in overlaying multiple patterns over a wafer. The throughput is the number of sheets processed per unit of time.
The fabrication of a device using the lithography technique has employed a projection exposure apparatus that uses a projection optical system to project a pattern drawn on a mask or reticle (these terms are used interchangeably in this application) onto a wafer, thereby transferring the pattern. The projection optical system enables diffracted beams from the pattern to interfere on a wafer and forms an image. The normal exposure enables 0-th order and ±1st order diffracted beams (namely, three beams) to interfere with each other.
Mask patterns include an adjacent and periodic line and space (L & S) pattern, a line of contact holes that are adjacent and periodic (i.e., arranged at the same interval as the hole diameter), isolated contact holes that are non-adjacent and isolated, other isolated patterns, etc., and a transfer of a pattern with high resolution requires a selection of optimal exposure conditions (such as illumination conditions, exposure light amount, etc.) in accordance with kinds of patterns.
The resolution R of a projection exposure apparatus is given in the following Rayleigh equation:R=k1(λ/NA)  (1)where λ is a wavelength of a light source, NA is a numerical aperture of the projection optical system, k1 is a constant determined by a development process and others. In a normal exposure case, k1 is approximately 0.5–0.7.
The recent demand for highly integrated devices have increasingly required more fine patterns to be transferred or higher resolution. Although the above equation reveals that the higher numerical aperture NA and reduced wavelength λ would effectively achieve the higher resolution, improvements of these factors have already reached the limit at the current stage. Thus, it is difficult for the normal exposure to form a pattern of 0.15 μm or less onto a wafer. Accordingly, it has been suggested to employ the phase shift mask technology that enables two beams out of those diffracted beams which have passed through a pattern to interfere with each other, thus forming an image. The phase shift mask reverses, by 180°, phases of adjacent light-transmitting portions on it, and cancels out the 0-th order diffracted beam, thus enabling two ±1st order diffracted beams to interfere with each other and forming an image. Use of this technique would reduce k1 in the above equation down to substantially 0.25, thus improving the resolution R and forming a pattern of 0.15 μm or less onto a wafer.
However, when adjacent phases are altered by 180° for fine contact holes near the resolution limit, light is diffracted at a wide angle from the optical axis, i.e., in a direction of 45° on the pupil plane and, and deviates from the pupil in the projection system. As a result, the diffracted light can neither pass the pupil in the projection lens nor resolve. What can resolve is, at best, a fine pattern down to square root 2 times a marginal critical dimension in the L & S. Therefore, a contact line of holes (or contact holes array) has been demanded to have resolution equivalent to that of the L & S pattern.
Moreover, the recent semiconductor industry has been shifting its production to system chips that include highly value-added and various types of patterns, and thus it has become necessary to form more than one kind of contact hole pattern on a mask. However, a prior art phase shift mask has not yet sufficiently exposed, at one time with high resolution, a contact hole pattern blended with a contact hole line and an isolated contact hole. It is, on the other hand, conceivable to use the double exposure (or multiple exposure) with two masks to expose different kinds of patterns separately, but the conventional double exposure requires two masks and incurs many practical disadvantages: That is, this approach results in an increased cost and lowered throughput because of two exposure steps, as well as requiring high overlay accuracy for two mask exchanges.